The present invention relates to an automatic gain control system and, more particularly, to an automatic gain control system used for a mobile communication unit adopting CDMA (Code Division Multiple Access) for multi-access using spread spectrum.
Hitherto, communication apparatuses, in particular, digital mobile communication terminal apparatuses use an automatic gain control system for controlling the gain of a transmission power by comparing a base-band signal as reference data to be generated by a transmit-receive control unit with detection data of a transmission output to obtain a stable transmission power.
Generally, a wave detector unit used when forming the detection data of the transmission power comprises a diode, a capacitor C, and a resistor R, etc. The wave detector unit detects an envelope for an input signal.
However, when an envelope of a modulated wave having a varying amplitude is detected for a non-modulated wave having a predetermined power, the power of the input signal might not be effectively converted into a voltage component if a suitable level of variation in charging/discharging determined by a time constant CR is not supplied. Therefore, for correct detection, it is necessary to set a proper time constant determined by the capacitor C and the resistor R.
However, in a system wherein a plurality of users use the same frequency, typically, e.g., in CDMA for mobile communication, power is amplified by controlling the power of a used signal on the basis of a user""s position. It is difficult to obtain the proper time constant CR corresponding to all output patterns of the modulated waves because the variation in level of the power is remarkable by multiplexing a plurality of signals to improve the availability of frequencies, thereby preventing stable detection.
The present invention is made in consideration of overcoming the above problems. It is an object to provide an automatic gain control system capable of compensating for a difference which is caused by a time constant during detection. According to the present invention, stable gain control is performed without being influenced by the remarkable variation in envelopes of a used signal.
More specifically, according to a first aspect of the present invention, in an automatic gain control system, a base-band signal as reference data of a transmission wave is multiplied by a time constant during detection, thereby compensating for the amount of deterioration in the transmission wave which is caused by the time constant during detection.
According to a second aspect of the present invention, an automatic gain control system includes a transmit-receive control unit for generating a base-band signal serving as reference data of a transmission wave, a radio frequency converter unit for converting a frequency of the base-band signal based on a gain variation control signal, a coupler unit for splitting the base-band signal whose frequency is converted, a wave detector unit for detecting one part of the split base-band signal, and a gain variation calculation circuit for outputting to the radio frequency converter unit the gain variation control signal which is generated based on a detection result by the wave detector unit and discrete data of the base-band signal outputted by the transmit-receive control unit. The wave detector unit outputs envelope data of the split part of the base-band signal and detected capacity information having a time constant of the wave detector unit to the gain variation calculation circuit as the detection result. Further, the gain variation calculation circuit has an A/D converter for converting the envelope data into discrete data, and a comparing calculation circuit for generating the gain variation control signal based on the result of comparing a value obtained by multiplying the discrete data of the base-band signal from the transmit-receive control unit by the time constant, as the detected capacity information, with the discrete data of the envelope data converted by the A/D converter.
According to a third aspect of the present invention, in the automatic gain control system, further, the transmit-receive control unit includes a serial-to-parallel converter for serial-to-parallel conversion of an input signal, an I/Q encoder for encoding the serial-to-parallel-converted signal by the serial-to-parallel converter to an I-channel signal and a Q-channel signal, a code for designating code information of the I-channel signal and the Qch signal encoded by the I/Q encoder, a modulator for modulating the I-channel signal and the Qch signal encoded by the I/Q encoder on the basis of the code information designated by the code, a power controller for controlling transmission powers of the I-channel signal and the Qch signal which are modulated by the modulator, at least two multipliers for controlling amplitude values of the I-channel signal and the Qch signal on the basis of the control of the transmission power by the power controller, a multiplexer for multiplexing the I-channel signal and the Qch signal from the multiplier, a filter for limiting bands of the Ich signal and the Q-channel signal from the multiplexer, a quadrature modulator for synthesizing and quadrature-modulating the Ich signal and the Q-channel signal whose bands are limited by the filter, a frequency converter for converting a frequency of the signal quadrature-modulated by the quadrature modulator into a desired frequency, and a D/A converter for converting the signal which is converted into the desired frequency by the frequency converter into an analog signal. The transmit-receive control unit outputs the signal quadrature-modulated by the quadrature modulator to the comparing calculation circuit as the discrete data of the base-band signal, and also outputs the analog signal converted by the D/A converter to the radio frequency converter unit as the base-band signal.
According to a fourth aspect of the present invention, in the automatic gain control system, the radio frequency converter includes a first mixer for upconverting the base-band signal from the transmit-receive control unit, a band-pass filter for removing an unnecessary wave other than a necessary frequency band from an output from the first mixer, a first amplifier for amplifying an output from the band-pass filter to a necessary power, a second mixer for upconverting the output amplified by the first amplifier, a variable attenuator for varying a gain of an output from the second mixer based on the gain variation control signal, and a second amplifier for amplifying an output from the variable attenuator to an output having a desired magnitude.
According to a fifth aspect of the present invention, in the automatic gain control system, the coupler unit directly outputs the base-band signal, without changing said signal, whose gain is controlled by the radio frequency converter unit based on the gain variation control signal as a transmission wave, and also supplies a signal by splitting 1/N of the transmission signal (N is an integer equal to 1 or more) to the wave detector unit.
According to a sixth aspect of the present invention, in the automatic gain control system, the comparing calculation circuit includes a time constant table in which the time constant, as the detected capacity information from the wave detector unit, is stored, a multiplier circuit for multiplying the discrete data of the base-band signal from the transmit-receive control unit by the time constant stored in the time constant table and for outputting discrete data, a comparator for comparing the discrete data from the multiplier circuit with the discrete data from the A/D converter, and a variable attenuator control unit for generating the gain variation control signal for controlling conversion by the variable attenuator based on a comparison result by the comparator.
According to a seventh aspect of the present invention, in the automatic gain control system, the comparator includes a first delay circuit for delaying the discrete data from the multiplier circuit by an arbitrary time, a second delay circuit for delaying the discrete data from the A/D converter by an arbitrary time, a first averaging circuit for averaging an output from the first delay circuit over a predetermined time, a second averaging circuit for averaging an output from the second delay circuit over the predetermined time, an adder for adding an output from the first averaging circuit and an output from the averaging circuit and outputting an average voltage value, a comparison value conversion table in which a comparison value for the average voltage value outputted from the adder is stored, and a multiplier for multiplying the average voltage value by the comparison value stored in the comparison value conversion table.
According to the present invention, by multiplying the base-band signal obtained by the transmit-receive control unit (control unit) by data equivalent to the time constant of the wave detector unit, the deterioration in the detection voltage is compensated. Incidentally, this deterioration in the detection voltage is caused in the case of detecting a modulation wave having a marked variation in amplitudes such as CDMA. Accordingly, it is possible to eliminate the variation in the detection voltage due to the type of signals inputted in the wave detector unit.